1. Field of the Invention
The invention is in the field of interactive communication and transport systems in which, on request, the information of still pictures specified in said request is forwarded to one or more attached workstations from a source of information of still pictures or of other bulk data. More specifically, the invention relates to an interactive communication and transport system in which communication for the purpose of requests between requesting workstations and at least one source of information of still pictures takes place via a relatively lowspeed communication path and the transmission, that is to say the actual transport, of information of still pictures specified in the request takes place from the source to the requesting workstations via a relatively high-speed transmission path.
2. Prior Art
Interactive communication and transport systems of a type as described above are known per se, for example from reference [1](see under C.). This reference describes a system for integrated communication services. Said system is composed of a first packetswitched ring-type network for communication services at relatively low speed, namely up to 10 Mbit/sec, and a second circuit-switched ring-type network, in overlay, for services which require a high speed. Bulk data transports take place via said overlay network which comprises eight high-speed channels of 70 Mbit/sec each, the access to the overlay network being controlled by means of communication at lower speed via the first network. One of the services which can be provided in such a network is, as described briefly in section 6 of said reference, the so-called `still picture` service. For this purpose, the system comprises one or more workstations and a source of requestable information of still pictures, which are attached both to the first network and to the overlay network. An attached workstation can send a request over the first network to the source for the purpose of forwarding a still picture specified in said request, whereupon the source then forwards said information to said workstation via a circuit- switched connection set up, to the requesting workstation in one of the high-speed channels reserved therefore.
Reference [2] discloses a digital picture communication network with two communication channels, of which a first channel handles general data communication and also acts as control channel for the second channel, over which the transport of digital bulk data exclusively takes place. Said bulk data may comprise information of still pictures, digitised speech information or other data in the form of large packets which are stored in a centrally situated database, the picture source, and which are requestable by a user from a workstation connected to both channels of the network by means of requests via the control channel. As is described in reference [2] from line 11 onwards on page 11 to line 16 inclusive on page 12, said known digital picture communication network is of a design such that, after the requested picture has been located in the picture source by means of indexing means, central control means first set up a circuit-switched connection to the requesting workstation over the second channel by means of communication over the first channel, whereupon the picture source is given permission to forward the requested picture data over said special connection. Once the transmission has been completed, the circuit-switched connection is disconnected, which is accompanied by the necessary control communication via the first channel, before a subsequent circuit-switched connection can be established for a subsequent request.
Said systems known from references [1] and [2] both have the disadvantage that a connection which is separate because it is circuit-switched always has to be set up to the requesting workstation for the purpose of forwarding the bulk data of a picture. Since the time period for which this connection exists is much longer than the period of the actual forwarding of a requested picture, the second channel is not used efficiently. This is all the more true if, in addition, forwarding of the same picture to yet one or more other workstations is necessary, since separate connections then have to be set up consecutively for this purpose. It is precisely in interactive applications, in which users must be able to request such pictures rapidly from their workstations, that this can result in unacceptable waiting times. It is furthermore not known from this how two or more picture sources can use one channel.
Reference [3] discloses a communication and transport system which comprises an overlay network which operates at relatively high speed and which is used in a packet-switched mode for transporting bulk data, in which the access to the overlay network is regulated with the aid of a single token mechanism on a packet-switched low-speed ring-type network. This system may comprise several bulk-data transmitters and many receivers which are connected to both networks. A receiver takes in a packet if the destination address in the header of the packet is recognised (page 16, line 20 - page 17, line 5). A transmitter transmits only after receiving and seizing a token circulating at set times under the control of a token manager, and releases said token after the packet, including the trailer, has been sent and a seized token has been received (page 16, lines 10-19).
A disadvantage of the system disclosed by reference [3] is that, since the token is always only released after the transmission of a packet, there always elapses, between consecutively transmitted packets originating from different transmitters, a certain time interval which, as stated, takes up on average approximately 10% of the total transmission time available, with the result that the overlay network cannot be used efficiently. In addition, this reference does not disclose whether and, if so, how a transmitter can forward, by means of a request originating from a workstation having a receiver attached to the network at some other point, bulk data specified in said request to said receiver.
Reference [4] discloses a transmission system for transmitting data packets of still pictures from a central station having a picture data source selectively over a transmission line to one or more of a number of attached local receiving stations. The selectivity is achieved as a result of a dynamic addressing by the central station with the aid of logical channel numbers in the data packets which are assigned in advance in separate commands to the receiving stations concerned via the same transmission line. This known system has the drawback that it is not an interactive system. In addition, the communication concerning the picture data transmission and the transmission itself take place over the same transmission line. In order not to hold one another up, the data packets have to be relatively small, and this entails a large overhead in the case of pictures of some size.